1. Field of the Invention
The invention is concerned with a process for the expansion of a hollow joining sleeve of a memory alloy.
The invention is further concerned with a device for the accomplishment of the process.
2. Description of the Prior Art
Hollow articles out of memory alloys can, because of their "shape memory" properties, be used as joining elements for semi finished products of all types such as wires, rods, tubes and profiles. For this purpose they must be expanded in the range of the low temperature phase, i.e. in the martensitic condition, by a certain amount which should not exceed a certain critical value. The current methods of stretching are known from the Standards (e.g. DIN 8585 sheet 3). It is further known (e.g. from DT-AS 20 65 651) that shrink fit joints can be made using a tube of a memory alloy. The workpieces to be joined were axially inserted into the tube which was expanded at low temperatures (such as &lt;-50.degree. C.), whereby the latter on subsequent warming (for example up to room temperature) shrunk and pressed itself fully against the workpieces.
In the conventional stretching processes the hollow article is clamped by some means around its circumference (e.g. using jaws), or its front face is pressed onto a jig. As the properties of memory alloys depend very sensitively on the extent and the direction of the total applied deformation, i.e. on their "history", additional uncontrollable stresses and deformations are produced in the workpiece through such a process, which impairs its functional proficiency.